1. Field of the Invention
This invention relates to serrated grating bars used in making platforms and walkways for large equipment and other industrial uses, and to methods and apparatus for making such grating bars.
2. Prior Art
Grating workstock is usually hot rolled low-carbon steel although aluminum or other suitable metals are sometimes used. The material comes in the form of coiled stripes which are not suitable for serrating, being distorted by the slitting and coiling operations. In addition to having edges which are not square and which may be burred, the hot rolled steel has a coating of hard mill scale which must be removed.
The present commercial state of the art is to feed the uncoiled strip into an apparatus called a straightener/leveler/flattener, for flattening and straightening. It is then passed through another apparatus known as an edger which squares the edges and reduces the burrs. The strip then progresses to a reciprocating punch press operating a cutting die. The die progressively punches out slugs of metal along one edge of the strip to produce the serrations. The serrated bar is then cut to length in other suitable equipment and the lengths are welded together in the desired configuration.
The above method of producing serrated grating bar requires a reinforced base to support the punch press and sufficient space to accommodate the press and other necessary equipment. In addition, this method produces scrap in the form of the blanks punches out by the press and these blanks must be a certain minimum size to preclude chipping of the cutting die. Furthermore, the process is noisy and leaves burrs of sharp edges on the walking surface of the grating bars which present a safety hazard in that a person could be cut if contact is made with the serrated surface of the grating bars.
U.S. Pat. No. l,636,592 suggests that grating bars can be made by passing straight bar stock between a toothed wheel and a grooved idler wheel to form depressions along one edge of the bar. The metal displaced flows transverse to the edge of the bar to form beads on either side of the depressions thereby providing increased contact area for securing the cross members which are seated in the depressions and welded. U.S. Pat. No. 3,653,245 suggests that serrated grating bars can be made from coiled strip in a similar manner by adding, after the toothed wheel, a straightener to remove the camber or curvature of the bar toward the notched edge, an edger to remove the burrs and square the edges and a flattener to remove the curvature about the longitudinal axis of the bar produced by notching and to roll out the metal displaced laterally during notching. The forces required to make the notches in the edge of the strip have a tendency to twist the bar about the longitudinal axis during notching, causing distortion in the serrations which is not removed and, in fact, is aggravated by the subsequent operations performed on the serrated bar. Furthermore, the rolling out of the metal displaced by the notches tends to cause excessive work hardening of the steel, resulting in a high ratio of yield strength/ultimate strength accompanied with a relatively low percent elongation which, in addition to the distortion in the serrations, detracts from the commercial appeal of the resultant product.